1. Field of the Invention
The present invention relates to analog to digital converters (ADC), and more specifically to a method and apparatus for calibrating capacitor mismatch in a pipe-line ADC.
2. Related Art
An analog to digital converter (ADC) generally refers to a component which converts an analog signal to a sequence of digital codes. In general, an ADC samples the analog signal at a time point specified by a clock signal, and generates a corresponding digital code depending on the voltage level of the sampled signal.
Pipeline ADC refers to a type of ADC in which multiple stages are connected sequentially. The output of each stage is provided as an input to the next stage, with the first stage receiving an analog signal as the input. Each stage may be logically viewed as resolving a portion (sub-code) of the digital code sought to be generated by the ADC, and generating an output signal which represents the unresolved portion.
Each stage of a pipeline ADC often contains capacitors (along with the operation of other components) to perform tasks such as sampling an input signal and generating the output signal representing the unresolved portion. For an ideal operation, the capacitance of the capacitors need to be in a desired ratio, but deviate from the desired ratios due to reasons such as imperfections in manufacturing processes and impact of change of operating conditions (e.g., temperature, voltage). Such deviations are generally referred to as a ‘Capacitor mismatch’.
One problem with capacitor mismatch is that each stage may not generate the output signal at accurate strength, which in turn leads to incorrect sub-codes being generated by stages down the processing path. Accordingly, it is desirable that the effects of mismatches be countered. One approach for such countering requires calibration of the mismatches. Calibration generally refers to measurement of the level of mismatch. Once the mismatch is calibrated, appropriate corrective action can be taken.
In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.